This invention relates generally to imaging methods and apparatus, and more particularly to methods and apparatus for imaging using x-ray tubes.
In computed tomography (CT) imaging systems, such as a CT imaging system performing cone beam imaging in a step-and-shoot mode of data acquisition, projection sampling is incomplete. There are two major sources of the incomplete sampling, which are the cone beam effect and longitudinal truncation. In particular, other than the center plane, exact reconstruction is not possible, which is the cone beam effect. Although many approximation algorithms are known to compensate for the cone beam effect, image artifacts become clinically unacceptable when the cone beam becomes large, such as when imaging larger regions (e.g., using a large image detector for an entire organ). Longitudinal truncation results because the coverage along the gantry rotation axis changes as a distance to the x-ray focal spot. Specifically, the coverage is much smaller for regions closer to the focal spot. As a result, a portion of the imaging volume to be reconstructed is not exposed to the x-ray radiation.
To overcome the cone beam and longitudinal truncation artifacts in, for example, a step-and-shoot mode CT, systems with multiple x-ray focal spots are known. Using two focal spot spaced along the z-direction (longitudinal direction of scanning) can significantly reduce the cone beam and truncation artifacts. Thus, it is desirable to provide an x-ray tube that produces two focal spots in the z-direction during data acquisition. During scanning, the two focal spots can be switched on and off alternatively such that each voxel in the reconstructed volume is irradiated from two different cone angles while at substantially the same projection view angle.